For this study, 340 children less than 18 months old from a low-income, urban neighborhood in Cali, Colombia, were observed from birth by means of weekly home visits to detect cases of acute respiratory tract infection. All suspected cases were confirmed by trained doctors in a special clinic. Information on symptoms, signs, and potential risk factors was documented prospectively. Etiologic agents were identified in cases of lower respiratory tract infection (LRI). The overall incidence of acute respiratory tract infection was 6.6 cases per child-year at risk. The incidence of upper respiratory tract infection was 4.9 cases per child-year at risk and that of LRI was 1.7 cases per child-year at risk. Crowding in the home was found to be significantly associated with an increased incidence of LRI. Respiratory syncytial virus was the viral agent most frequently isolated from cultures of nasopharyngeal aspirates of children with LRI. Staphylococcus aureus was the bacterial agent most frequently isolated from the blood of patients with LRI.